The present invention relates to electrical measurements and, in particular, to source measure units.
The use of source measure units (SMUs) has become common in many fields, particularly in the testing of semiconductors, integrated circuits and electronic devices.
Referring to FIG. 1, a basic prior art SMU circuit 10 is illustrated. In understanding these circuits it is important to note that the op-amps will do everything in their power to force the difference between their inputs to zero. If S1 is as shown, a voltage corresponding to VV will be forced across ZLoad. If S1 is moved to the other position, a current corresponding to VI will be forced through ZLoad (i.e., VI/RSense is the current through RSense and hence through ZLoad. The unforced parameter, current or voltage with respect to ZLoad, can then be measured with unshown measuring equipment.
The voltage across RSense is riding on top of the voltage across ZLoad. As a result, the amplifier 12 must not only deal with the voltage across its inputs, but also with the offset from ZLoad. In real-world amplifiers, the common mode errors introduced in translating the voltage from such an offset to ground reference are often the dominate errors in the circuit.